This invention relates to a process for the purification of crude terephthalic acid which is contaminated with reducible impurities in order to obtain a very pure fiber-grade acid.
Usually, terephthalic acid is produced by a liquid phase oxidation of p-xylene and/or p-toluic acid. Terephthalic acid is of great commercial importance and is widely used for the production of various different polymers, such as fiber-forming polyesters. A process for preparing polyesters of terephthalic acid, particularly polyethylene terephthalate, comprises a direct condensation of terephthalic acid with the respective polyalcohol, for example, terephthalic acid is reacted with ethylene glycol to form bis-(.beta.-hydroxyethyl) terephthalate which is then polymerized in a second stage. This direct condensation process is simpler than other known methods such as transerterification of dimethyl terephthalate with the appropriate glycol. However, the direct esterification requires the use of highly purified terephthalic acid. In order to be suitable for the production of polyester fibers, terephthalic acid must be substantially free of any contaminants which lower the melting point of the polyester and/or cause coloration of the polyester. In fact, some impurities which are contained in crude terephthalic acid are color-forming precursors of the terephthalic acid.
All these ipurities have not yet been identified, however 4-carboxybenzaldehyde which is an intermediate oxidation product and which in the following is abbreviated as 4-CBA, generally is found in crude terephthalic acid. It is known that the degree to which coloration in the polyester is induced is less if the 4-CBA-content of the terephthalic acid is low. While pure 4-CBA itself does not necessarily promote coloring during polymerization, this impurity is a convenient tracaer for evaluating the degree to which terephthalic acid has been refined. A process which can reduce the 4-CBA-content of terephthalic acid reduces also the content in color-forming precursors.
From the U.S. Pat. No. 3,584,039 it is known that fiber-grade terephthalic acid may be prepared by purifying crude terephthalic acid by means of a reduction procedure. The process is essentially comprised of treating an aqueous solution of crude terephthalic acid with hydrogen in the presence of a supported or unsupported Group VIII noble metal catalyst, whereby the metal and the support are insoluble in the solution under the working conditions. By this process, the amounts of 4-CBA and other coloring impurities contained in terephthalic acid are reduced under formation of removable products. Purified terephthalic acid is then recovered by crystallization, filtration and drying.
The development of the process for producing polyesters by direct condensation of terephthalic acid with ethylene glycol depends largely on the purity and price of the acid. There is thus a need for a purification process which produces very pure terephthalic acid and which can be carried out in a more economical manner than the previously used methods.